The present invention relates to a detection system for low flying aircraft and specifically to a combination of seismic and acoustic sensing capable of detecting low flying aircraft by correlating their seismic and acoustic signatures.
Conventional radio frequency (r.f.) radar systems possess certain natural limitations in the detection of low flying aircraft. Terrain echo returns have increasingly greater tendencies to obscure tracking of aircraft as they approach the ground. Certain aircraft will intentionally take advantage of this limitation and fly low to avoid the radar of law enforcement agencies, border patrol officials, and military facility protection systems.
Several alternatives to radar systems have already been tried. Past attempts to detect aircraft with microphones by analyzing aircraft acoustic signatures have suffered from false alarm problems caused by other sources of acoustic noise such as motor vehicles and wind. The use of geophones alone has been subject to false alarms caused by other sources of seismic signals.
Another solution to the low flying aircraft detection problem is the use of both a geophone and a microphone in a sensing system. This task has been alleviated to some degree by prior art techniques. The extent of these prior art techniques is given by the following three technical reports: Sandia Laboratories, September 1973, C. L. Schuster and J. E. Scheibner, "A Low-cost-to-produce Unattended Ground Sensor", J. P. Claassen and M. M. Patterson, "A Comparitive Study of Adaptive Noise Cancellation Algorithms for Intrusion Detection Systems", Proceedings of the 24th Midwest Symposium, June 29-30, 1981, Albuquerque, NM, B. Widrow, et al, "Adaptive Noise Cancelling: Principles and Applications", Proc. IEEE, Vol. 63, No. 12, December 1975, pp. 1692-1716.
One known approach consists of a system of detecting personnel, land vehicles, propeller and jet aircraft thru the use of a geophone and a microphone. The above detection system accomplishes classification by a comparison of energy levels between the seismic and acoustic signals occuring in certain frequency bands (e.g. the seismic signal of a land vehicle or a pedestrian at 50H.sub.z is presumed to be greater in magnitude than its acoustic signal; while an aircraft acoustic signal at 50H.sub.z is presumed to be greater in magnitude than its seismic signal etc.)
While the classifier logic of the prior art detection system described above is successful in distinguishing between land and air vehicles under a limited set of conditions, it remains inadequate for purposes of the detection of aircraft in that it remains subject to false alarms in the form of other types of acoustic signals which would satisfy the classifier logic requirements yet are not aircraft.
The prior art device compares the energy levels of seismic and acoustic signals and classifies the source of the signals using the classification logic partially described above. The subject invention incorporates the advent of two new phenomena into an aircraft detection principle. First, it has been determined that the seismic and acoustic signatures for low-flying aircraft have substantial correlation at large target-to-sensor distances. Second, it has been determined that there exists extremely little correlation between the seismic and acoustic signals generated by wind, seismic events, and land vehicles in motion.
The technical reports of both Widrow et al, Claassen and Patterson describe prior art alternative noise cancellation algorithms and devices which may be successively used as one element of the subject invention: the adaptive noise canceller. The detection of aircraft by determining the degree of correlation between seismic and acoustic signals and the application of an adaptive noise canceller as a means to that end is in a manner different from the prior art.
In view of the foregoing, it is apparent that there currently exists the need for a detection system of low-flying aircraft which senses the presence of aircraft by the phenomenon of the correlation between their seismic and acoustic signals. The present invention is directed towards satisfying that need.